1. Field of the Invention
This invention relates to a method for producing a sintered body of high density hexagonal boron nitride (hereinafter referred to simply as "boron nitride"), which is a ceramic material excelling in resistance to heat, resistance to thermal shock, lubricity, corrosionproofness, electric insulating property, etc.
2. Prior Art Statement:
Boron nitride is a ceramic material having many outstanding properties i.e. excelling in thermal, chemical and electrical properties, possessing lubricity, and permitting easy machinability.
Thermally, it manifests stability in an inert atmosphere at temperatures up to about 3,000.degree. C., exhibits extremely high thermal conductivity and great resistance to thermal shock. It also excels in chemical stability sufficiently to be less liable to be wet with and avoid reacting with a molten metal. Thus, it finds extensive utility in application to heat-resistant and corrosionproof materials. Since it excels in lubricity as well as in thermal stability, it constitutes a valuable material for high-temperature lubricating products.
Further, since it exhibits extremely high electric resistance and undergoes only nominal change even at highly elevated temperatures, it forms an electrically insulating material usable over a wide range of temperatures.
Boron nitride, which excels in thermal and chemical properties, is so difficult to sinter that the production of a sintered body of this compound necessitates a mechanical treatment at highly elevated temperatures under high pressure. In making sintered bodies of boron nitride on an industrial scale, there is employed, for example, a method of hot pressing a fed material of boron nitride that contains B.sub.2 O.sub.3 unreacted at the time of preparing the fed material of boron nitride or a method which accomplishes the production by mixing boron nitride powder with several to ten-odd percent of borates of alkaline earth metals such as, for example, MgO.multidot.B.sub.2 O.sub.3, CaO.multidot.B.sub.2 O.sub.3 and SrO.multidot.B.sub.2 O.sub.3 as a binder, packing the resultant mixture in a graphite die, and heating the mixture and the enclosing graphite die in a high-frequency induction heating furnace under application of pressure in the range of 200 to 400 kg/cm.sup.2 at a temperature in the neighborhood of 2,000.degree. C. (Japanese Patent Publication SHO 49(1974)-40124).
The present inventors formerly developed a method for the production of a sintered body of boron nitride containing only a small proportion of binder (Japanese Patent Public Disclosure SHO 59(1984)-162179). This method relies for sintering exclusively on the use of a hot press.
Japanese Patent Publication SHO 40(1965)-10422 discloses a method for the production of a body having a material of a low Young's elastic modulus uniformly distributed in a continuous phase formed of a material of a high Young's elastic modulus by mixing a dense refractory material of high Young's elastic modulus selected from the group consisting of the carbides of the metals of Groups IIIA, IVA, VA and VIA of the Periodic Table of Elements, the borides of the metals of Groups IVA, VA and VIA of the Periodic Table of Elements, the nitrides of the metals of Groups IIIA and IVA of the Periodic Table of Elements, the silicides of the metals of Groups IVA, VA and VIA of the Periodic Table of Elements, the oxides of the metals of Groups IIA, IIIA and IV of the Periodic Table of Elements, and the carbide, boride, nitride, and oxide of silicon with a refractory material of graphite or boron nitride having a low Young's elastic modulus and exposing the resultant mixture to the actions of high temperature and high pressure.
Of the conventional methods described above, the method which uses boron nitride containing unaltered B.sub.2 O.sub.3 as a starting material for hot pressing and a method which uses an alkaline earth metal salt in a large amount as a binder have the following disadvantage.
The sintered bodies obtained by these conventional methods cannot fully manifest the outstanding characteristics inherent in boron nitride because they contain several to ten-odd percent of oxides, predominantly of B.sub.2 O.sub.3. Though boron nitride itself is stable at elevated temperatures up to about 3,000.degree. C. in an inert atmosphere, the sintered body containing oxide binder predominantly of B.sub.2 O.sub.3 has a very severely limited working temperature because the binder spurts out of the sintered body at a temperature of one thousand several hundred degrees centigrade, reacts with other materials coming in contact with the sintered body, diffuses in the ambience and pollutes the immediate environment, and causes cracks in the sintered body of boron nitride.
When boron nitride free from the aforementioned disadvantage due to the inclusion of a binder of a relatively low melting point predominantly of B.sub.2 O.sub.3 can be produced by a pressureless sintering method, a sintered body of a heretofore unattainable high quality ought to be obtained inexpensively.
The aforementioned method developed by the present inventors and disclosed in Japanese Patent Public Disclosure SHO 59(1984)-162179 uses a binder only in a small amount but requires exclusive use of a hot press for the purpose of sintering.
The refractory article produced by the method disclosed in Japanese Patent Publication SHO 40(1965)-10422 invariably contains graphite when it contains a continuous phase of boron nitride. Where boron nitride is present in the form of a dispersed phase, there occurs a continuous phase formed of a valuable combination of zirconium boride and molybdenum disilicide. Thus, in the refractory article as a finished product, the content of boron nitride is lowered. Further, since the sintering is not carried out in an atmosphere of nitrogen, any boron carbide contained in the raw material is not converted into boron nitride.
An object of this invention is to provide a method for producing a high purity boron nitride sintered body with high density and high strength easily under a pressureless condition, optionally omitting the use of a hot press.